Microlithography (also referred to as photolithography or simply lithography) is a technology for the fabrication of integrated circuits, liquid crystal displays and other microstructured devices. The process of microlithography, in conjunction with the process of etching, is used to produce patterns in thin film stacks that have been formed on a substrate, for example a silicon wafer. At each layer of the fabrication, the wafer is first coated with a photoresist which is a material that is sensitive to electromagnet radiation. Next, the wafer with the photoresist on top is exposed to projection light through a mask in a projection exposure apparatus. The mask contains a circuit pattern to be projected onto the photoresist. After exposure the photoresist is developed to produce an image corresponding to the circuit pattern contained in the mask. Then an etch process transfers the circuit pattern into the thin film stacks on the wafer. Finally, the photoresist is removed. Repetition of this process with different masks results in a multi-layered microstructured component.
A projection exposure apparatus generally includes an illumination system, a mask alignment stage for aligning the mask, a projection objective (sometimes also referred to as ‘the lens’) and a wafer alignment stage for aligning the wafer coated with the photoresist. The projection objective images, usually at a reduced scale, a portion of the circuit pattern, which is illuminated on the mask by the illumination system, on the photoresist.
One of the main aims in the development of microlithographic projection exposure apparatus is to be able to lithographically define features with smaller and smaller dimensions on the wafer. Small features lead to a high integration density, which generally has a favorable effect on the performance of the microstructured components that are produced with the aid of such apparatus.
The minimum size of the features that can be lithographically defined is approximately proportional to the wavelength of the projection light. Therefore the manufacturers of such apparatus strive to use projection light having shorter and shorter wavelengths. The shortest wavelengths currently used are 248 nm, 193 nm and 157 nm and thus lie in the deep ultraviolet (DUV) or vacuum ultraviolet (VUV) spectral range.
The next generation of commercially available apparatus is expected to use projection light having an even shorter wavelength of about 13.4 nm which is in the extreme ultraviolet (EUV) spectral range. Because there are generally no optical materials available that are transparent for EUV light, it is not generally possible to use lenses or other refractive optical elements in such an apparatus. Instead, the optical systems of such an apparatus are typically catoptric, which means that all optical elements (including the mask) have to be reflective.
The illumination system of an EUV projection exposure apparatus typically includes one or more multi facet mirrors. A multi facet mirror includes a plurality of mirror members (occasionally referred to as mirror facets) each having a flat or curved reflecting surface. Sometimes the EUV illumination system includes one multi facet mirror that is used to determine the intensity distribution in a pupil surface of the illumination system. This intensity distribution, in turn, determines from which direction EUV projection light impinges on the mask. Changing these directions involves tilting the mirror members of the multi facet mirror. Another multi facet mirror is often used to produce a plurality of secondary light sources that commonly illuminate the mask. In principle, a multi facet mirror may also be used in the projection objective, for example at a position in or in close proximity to a pupil surface.
Some multi facet mirrors include mirror members which can be adjusted individually with the help of actuators. Such a multi facet mirror is described in U.S. 2005/0030653 A1. Each mirror member of this prior art multi facet mirror has a substrate which has at least approximately the shape of a truncated ball. The truncation defines a flat or curved area on which a reflective coating is applied. The substrate is connected to an actuating rod having a longitudinal axis which coincides with an axis of symmetry of the ball-shaped substrate. A support plate of the multi facet mirror is provided with a plurality of sockets for the ball-shaped substrates. The actuating rod extends through the socket and projects from a bottom surface of the support plate. This projecting portion of the actuating rod is connected to an actuator which is configured to move the actuating rod laterally. Lateral displacements of the free end of the actuating rod cause tilting movements of the entire mirror member.
U.S. 2009/0225297 A1 describes an actuator for a lens that includes three actuator members which are evenly distributed along the circumference of the lens. Each actuator member includes three piezoelectric stacks that are arranged one on top of the other. One stack is capable of changing its length along one direction, and the other two stacks are capable of performing shearing deformations along two further directions. If all these directions are orthogonal to each other, the lens can be moved along arbitrary directions and also be tilted around certain tilting axes.
WO 2010/037476 A2 describes the use of ultrasonic transducers to tilt mirror members of a multi facet mirror.
If the density of the mirror members in such multi facet mirrors is very high, the volume which is available for accommodating a single actuator becomes very small. Particularly if the range of possible tilting angles shall be large, it is difficult to arrange an actuator at one side of the actuating rod within a volume that is laterally confined (at least approximately) by the circumference of the mirror substrate.